Such fluid working machines are generally used, when fluids are pumped or a fluid is generating a mechanical movement. The word “fluid” can relate to both gases and liquids. Of course, the word “fluid” can even relate to a mixture of gas and liquid and furthermore to a supercritical fluid, where no distinction between gas and liquid can be made anymore.
In particular, such fluid working machines are used, if the pressure level of a fluid has to be increased. For example, such a fluid working machine could be an air compressor or a hydraulic pump.
Generally speaking, fluid working machines comprise one or more working chambers of a cyclically changing volume. Usually, for each cyclically changing volume, there is provided a fluid inlet valve and a fluid outlet valve.
Traditionally, the fluid inlet valves and the fluid outlet valves are passive valves. When the volume of a certain working chamber increases, its fluid inlet valve opens, while its fluid outlet valve closes, due to the pressure differences, caused by the volume increase of the working chamber. When the volume of the working chamber decreases again, the fluid inlet valve closes, while the fluid outlet valve opens due to the changed pressure differences.
A relatively new and promising approach for improving fluid working machines are the so-called “synthetically commutated hydraulic pumps”, also known as “digital displacement pumps”. Such pumps are a subset of variable displacement pumps. Such synthetically commutated hydraulic pumps are known, for example, from EP 0 494 236 B1 or WO 91/05163 A1. In these pumps, the passive fluid inlet valves are replaced by electrically actuated inlet valves. Preferably the passive fluid outlet valves are also replaced by electrically actuated outlet valves. By appropriately controlling the valves, a full-stroke pumping mode, an empty cycle pumping mode (idle mode) and a part-stroke pumping mode can be achieved. Furthermore, if both inlet and outlet valves are electrically actuated, the pump can be used as a hydraulic motor as well. If the pump is run as a hydraulic motor, full-stroke motoring and part-stroke motoring is possible, as well.
A major advantage of such synthetically commutated hydraulic pumps is their higher efficiency, as compared to traditional hydraulic pumps. Furthermore, because the valves are electrically actuated, the output characteristics of a synthetically commutated hydraulic pump can be changed very quickly.
For adapting the fluid flow output of a synthetically commutated hydraulic pump according to a given demand, several approaches are known in the state of the art.
It is, for example, possible to switch the synthetically commutated hydraulic pump to a full pumping mode for a certain time. When the synthetically commutated hydraulic pump is pumping, a high pressure fluid reservoir is filled. Once a certain pressure level has been reached, the synthetically commutated pump is switched to an idle mode and the fluid flow demand is supplied by the high pressure fluid reservoir. As soon as the high pressure fluid reservoir has reached a certain low pressure threshold, the synthetically commutated hydraulic pump is switched on again.
This approach, however, necessitates a relatively large high pressure fluid reservoir. Such a high pressure fluid reservoir is expensive, occupies a large volume and is quite heavy. Furthermore, a relatively large variation in the output pressure will inevitably occur.
So far, the most advanced proposal for adapting the output fluid flow of a synthetically commutated hydraulic pump according to a given demand is described in EP 1 537 333 B1. Here, it is proposed to use a combination of an idle mode, a part-stroke pumping mode and a full-stroke pumping mode. In the idle mode, no effective pumping is done by the respective working chambers during their working cycle. In the full-stroke mode, all of the usable volume of the working chamber is used for pumping. In the part-stroke mode, only a part of the usable volume is used during the respective working cycle. The different modes are distributed among several chambers and/or several successive cycles in a way, that the time averaged effective flow rate of fluid equals the demand.
In EP 1 537 333 B1, for the part-stroke mode, a certain small fraction of the available volume is chosen. The fraction is chosen beforehand and fixed to a certain number in the controlling unit when the hydraulic pump is operating. This limitation to a certain number has been done for practical purposes: For this number, a calibrating measurement was performed. I. e., for a volume fraction of for example 16%, the actual value of the corresponding actuation angle/firing angle of the fluid inlet valve was determined. The angle has to be established by measurement to get a precise connection between firing angle and fluid flow and thus a precise fluid flow control ability. It has to be noted, that even for hydraulic oil, the compressibility is in the order of a few percent of the total volume at standard hydraulic oil pressures of 300 bars to 500 bars.
While the method, described in EP 1 537 333 B1 is an improvement over previous methods, it still suffers from several drawbacks. For the following examples let's assume that the partial stroke is set to 16%. This number has been chosen by the applicant of EP 1 537 333 B1 for their pumps.
If the fluid flow demand is very low (e.g. 2%) the time interval between two consecutive pumping strokes is still very long even if part-stroke pumping is performed. This can cause a severe pressure pulsation problem. Also, a start-stop (“stiction”) effect can be noticed at the load.
A similar problem arises, if the fluid flow demand is slightly above the number of the volume fraction chosen for partial strokes. If the fluid flow demand is for example 17%, the best way to comply with this demand is to use a series of spaced full-stroke pulses. However, the time between two consecutive full-stroke pulses is quite large. This again can cause severe pulsation problems as well as a “stiction”-effect. It should be noted, that there is no advantage, if some part-stroke pulses are performed between two full-stroke pulses. In this case, the time interval between two full-stroke pulses would be even larger.
Another problem occurs, if the fluid flow demand comes close to the full-stroke pumping capacity of the hydraulic fluid pump. Here, a full-stroke will be missing only once in a while. To be exact, the fluid flow will be a series of full-stroke pulses with infrequent, but significant holes of missing full-stroke pulses. This again can cause severe problems with pressure pulsations.